Screw head

ABSTRACT

Fastening system including a screw component ( 10 ) and a seating component ( 20 ). The screw component having a shaft ( 12 ) of maximum radius R 1 , a threaded section ( 13 ), and a head ( 14 ) having a bottom end ( 15 ) having a larger maximum radius R 2  than the screw shaft such that an exposed underside ( 18 ) of the screw head extends radially beyond the maximum radius of the screw shaft. The system further includes a seating component with a screw channel ( 21 ) and a planar screw seat ( 22 ) having a minimum radius R 3  which is equal to or greater than R 1 . The bottom end of the screw head has an annular contact surface for abutment against the screw seat, the contact surface being formed by the distal end of at least one downwardly extending protrusion ( 17 ) on the underside of the screw head, wherein the annular contact surface has a minimum radius greater than the minimum radius of the screw seat.

This invention relates to screws, particularly those used in the dentalimplant field to attach secondary components, such as abutments, to adental implant.

Dental implants are used to replace individual teeth or for anchoringmore complex structures, which generally replace several or even all ofthe teeth.

Implants are often constructed in two parts, in which case they consistof an anchoring part, often referred to in isolation as the implant, andof a separate abutment. The anchoring part is either embedded completelyin the bone, that is to say to the height of the alveolar crest, orprotrudes by a few millimetres from the alveolar crest into the softtissue. The abutment is mounted on the anchoring part and extends intothe oral cavity to form a support for a dental prosthesis or denture.

During the lifetime of the prosthesis, which can be over 20 years, theimplant system will be subjected to large loads caused by mastication.The abutment must therefore be firmly fastened to the implant in orderto prevent loosening and potential loss of the component. This can beachieved in numerous ways, for example, via compression fit or gluing.However, screw fit connections are generally preferred. By applying asufficiently high torque during attachment a firm connection between theimplant and abutment can be achieved.

In many systems therefore the implant comprises an internal threadedbore, while the abutment comprises a corresponding apical thread, thusallowing the abutment to be screwed directly into the implant.

However, this has the disadvantage that the exact angular position ofthe abutment relative to the implant is not known until final fixation.This can have disadvantages, particularly when the abutment is intendedto support a single tooth prosthesis.

Therefore, many implant systems comprise anti-rotation means, whichprevent relative rotation between the implant and abutment and which seta finite number of angular positions which the abutment can haverelative to the implant.

These anti-rotation means consist of complementary non-circularsymmetric portions in the implant and abutment, usually having apolygonal shape such as a hexagon or octagon.

Such systems ensure that the exact angular position of the abutment inrelation to the implant is known prior to fixation and can help toprevent loosening of the abutment during the lifetime of the implant.

Of course, when such anti-rotation means are employed it is not possibleto rotate the abutment relative to the implant and hence the abutmentcan no longer be directly screwed into the implant. Therefore a thirdcomponent, often a screw known as a “basal screw”, is used to connectthe abutment to the implant.

When a basal screw is used the abutment typically comprises a screwchannel extending through the abutment and having a screw seat. Thisenables the basal screw to be fed through the abutment until the screwhead abuts the screw seat and for a screwdriver to be inserted into thechannel to engage the screw and fasten this to the internal threadedbore of the implant, thus clamping the abutment securely to the implant.

An example of such a known implant system can be found for instance inEP1679049, in which the screw seat is conical and WO2006/012273, inwhich the screw seat is planar.

As well as abutments, basal screws are also used to connect other,temporary secondary components to the implant, for example, healingcaps, closure screws and impression posts.

As mentioned above, it is important that the abutment in particular isfirmly fastened to the implant in order to prevent loosening over thelifetime of the implant system. In the case of screw fit systems, thisis achieved by tightening the screw component, whether this is a thirdcomponent or the abutment itself, in order to achieve a high pre-load,or clamping force.

In order to achieve the maximum pre-load, it is desirable to tighten thescrew as much as possible without reaching the yield strength of thescrew. At this point, the tension within the screw body can result inplastic deformation of the threads and in some cases fracturing of thescrew. This is highly undesirable as the screw must then be removed andreplaced. Removal of a damaged screw is not always easy and furthermorethis can result in damage to the internal threads of the implant. Insome cases, the damage to the screw may not become apparent until afterthe final prosthesis has been fixed to the abutment, and hence thereplacement of the screw can also result in the need for the creation ofa new prosthesis.

Manufacturers of dental implant systems therefore set recommendedmaximum torque values, which ensure a high pre-loading of the screwwithout risking over tensioning. However, given the natural desire toensure a high pre-loading of the implant system, dental practitionersoften apply a fastening torque significantly over the recommended value,which can lead to failure of the screw.

In order to increase the tensional strength of screws to preventbreakage in such situations, one potential solution would be tomanufacture the screws from a different, stronger material. However,given the long term use of the screw within the human body any newmaterial must undergo rigorous safety tests, and finding a new materialhaving the necessary high strength together with the requiredbiocompatibility is not a simple matter.

Another option would be to increase the dimensions of the screw.However, in dental implant systems space is restricted as the implantmust fit within the available space within the jaw bone whilst removingas little bone mass as possible, to limit trauma at the implant site.Therefore the overall dimensions of the implant system can not bealtered, and so any increase in the diameter of the screw would resultin an equivalent reduction in the thickness of the implant and/orabutment. Such a modification would simply weaken the system in anotherarea.

It is therefore an object of at least a preferred embodiment of thepresent invention to provide a screw component having a design whichenables this to withstand a higher amount of torque without requiring achange of material or overall dimensions.

In accordance with one aspect the present invention provides a fasteningsystem comprising a screw component and a seating component. The screwcomponent comprises a shaft extending along a longitudinal axis andhaving a maximum radius R₁, said shaft comprising a threaded section,said screw component further comprising, at one end of the screw shaft,a head, the head comprising a bottom end having a larger maximum radiusR₂ than the screw shaft such that an exposed underside of the screw headextends radially beyond the maximum radius of the screw shaft. Theseating component comprises a screw channel comprising a planar screwseat having a minimum radius R₃ which is equal to or greater than R₁.The bottom end of the screw head comprises an annular contact surfacefor abutment against said screw seat, the contact surface being formedby the distal end of at least one downwardly extending protrusion on theunderside of the screw head, wherein the annular contact surface has aminimum radius greater than the minimum radius of the screw seat.

In the present context the “bottom” of the screw component is consideredto be the distal end of the screw shaft, i.e. the opposing end of thescrew shaft to the screw head. The bottom end of the head is thereforethe end of the head closest to the screw shaft and a downwardlyextending protrusion is one that protrudes towards the bottom of thescrew.

In accordance with conventional dental terminology, “apical” refers tothe direction towards the bone and “coronal” to the direction towardsthe teeth. Therefore the apical part of a component is the part which,in use, is directed towards the jaw bone and the coronal part is thatwhich is directed towards the oral cavity. When the screw component ofthe present invention is a dental screw component therefore, the bottomend of the screw head can also be considered as the apical end and theone or more protrusions as apically extending.

According to the present invention, the annular contact surface of thescrew head is located at a radial location remote from the outer radiusof the screw shaft. In other words, at the axial location of the annularcontact surface a gap exists between the minimum radius of the contactsurface and the radially inner part of the screw head. This is achievedby providing, on the underside of the screw head, one or moreprotrusions which extend in the downward direction to create the annularcontact surface. The gap created between the inner portion of the screwhead and the annular contact surface is great enough that the annularcontact surface has a larger minimum radius than the minimum radius ofthe screw seat.

As the minimum radius of the contact surface is greater than the minimumradius of the screw seat, the innermost area of the screw seat will notbe in contact with the screw head during use.

When tightening a screw only some of the applied torque is translatedinto pre-load. The overall torque which must be applied to the screw issignificantly higher, as a large amount of torque is used to overcomethe friction acting on the screw head and threads. In general it isestimated that only approximately 10-15% of the applied torque is usedto tighten the screw.

In prior art screw systems the underside of the screw head is shaped tocomplement the screw seat. Therefore, when the screw seat is planar theunderside of the screw is also planar. Thus, a large contact surface isformed. In effect, the entire underside of the screw head acts as anannular contact surface. In contrast, in the present invention, the atleast one downwardly extending protrusion creates a smaller contactsurface as only a part of the underside of the screw head forms thecontact surface. The protrusion has the effect that not all of theradially overlapping areas of the screw head and screw seat are incontact with one another. As the annular contact surface has a minimumradius greater than the minimum radius of the screw seat, this has alarger friction radius than is achieved when using a traditional flatscrew head, which also contacts the inner part of the screw seat.Consequently, the torque required to overcome the friction on the screwhead is increased and hence reduces the percentage of torque translatedinto pre-loading force.

Therefore, a user who exceeds the recommended maximum torque limit isless likely to damage the screw as more of the applied torque is“absorbed” by the frictional resistance of the screw head.

In most systems, the minimum radius of the screw seat is equal to themaximum radius of the screw shaft, taking into account manufacturingtolerances. This provides the narrowest screw channel possible below thescrew seat which can still accommodate the screw shaft. Consequently,traditional flat head screws comprise a planar underside that extendsoutwards from R₁.

Therefore, viewed from another aspect the present invention provides ascrew component comprising a shaft extending along a longitudinal axisand having a maximum radius R₁, said shaft comprising a threadedsection, said screw component further comprising, at one end of thescrew shaft, a head, said head comprising a bottom end having a largermaximum radius R₂ than the screw shaft such that an exposed underside ofthe screw head extends radially beyond the maximum radius of the screwshaft, said bottom end comprising an annular contact surface forabutment against a planar screw seat, the contact surface being formedby the distal end of at least one downwardly extending protrusion on theunderside of the screw, said contact surface having a minimum radiusgreater than the maximum radius of the screw shaft.

Preferred features of both aspects of the invention are described below.

The annular contact surface can be formed by a plurality of protrusions,such that the contact surface is discontinuous, or “broken”. When theunderside of the screw head comprises multiple protrusions, the distalends of these may be at different radial locations. For example, thedistance from the longitudinal axis to the protrusions may alternatefrom protrusion to protrusion. In such embodiments the minimum radius ofthe annular contact surface is set by the distal end of theprotrusion(s) closest to the longitudinal axis. However, preferably thedistal ends of the plurality of protrusions are an equal distance fromthe longitudinal axis. In addition, although the protrusions may havediffering shapes, it is preferred that these are identical, or at leastthat the shape of their distal ends are identical.

Preferably however the annular contact surface is formed by a singleprotrusion extending 360° about the longitudinal axis. Preferably thisannular contact surface has a uniform inner radius, although it ispossible for the annular contact surface to have an irregular, e.g.undulating, shape. Preferably the annular contact surface is uniformabout the longitudinal axis, i.e. the minimum and maximum radii are bothuniform. As the contact surface is intended to abut, in use, the planarscrew seat, the annular contact surface lies in a plane perpendicular tothe longitudinal axis.

The provision of a contact surface having a uniform inner radius enablesthe greatest friction radius to be achieved. The “friction radius” isthe mean radius of the contact surface. This is the surface which in usecontacts the screw seat and hence the greater this radius the greaterthe torque required to overcome the frictional resistance under thescrew head.

Traditional methods of increasing the friction radius include wideningthe screw head and/or increasing the minimum radius of the screw seat.However, as discussed above, in certain systems, such as dental implantsystems, where space is restricted, such increases are not possible orwould lead to unacceptable weakening of the components clamped by thescrew.

The present invention provides an alternative way of increasing thefriction radius which does not require any loss of volume from thesurrounding components. Instead, this can be achieved through arelatively minor modification to the underside of the screw head.

In principle it is preferable for the annular contact surface to be asnarrow as possible and have a minimum radius that is as large aspossible, in order to maximise the friction radius. For this reason itis also preferred that the maximum radius of the annular contact surfaceis equal to the maximum radius of the bottom end of the screw head. In apreferred embodiment the maximum radius of the annular contact surfaceis equal to the outer radius of the screw head. However, in practicemanufacturing and other concerns must also be taken into account.

For example, the screw seat can be located at the end of the screwchannel, such that the screw seat is formed on an exterior surface ofthe seating component. However, in many cases, the screw seat will belocated within the screw channel. In such cases the screw channel isformed of at least two sections, a first section, having a firstdiameter, and a second section having a second, smaller diameter,wherein the screw seat is formed by the transition between these twodiameters. In some components this transition may happen gradually,leading to a conical screw seat. However, this invention is onlyconcerned with planar screw seats, where at least a part of thetransition between the first and second diameters happens as a stepchange. At the transition between the wall of the first screw channelsection and the screw seat a small radius is often formed, due to themanufacturing methods used to create this channel. When the screw headhas an outer radius which is approximately equal to the diameter of thefirst screw channel section, an annular contact surface located at theradial edge of the screw head may not sit correctly on the screw seatand may further not be capable of smooth rotation.

In addition, depending on the function of the screw component, the headmay taper radially outwards from the bottom end, such that the maximumradius of the screw head is significantly greater than the screw seatradius and hence the contact surface radius.

Therefore, alternatively the location of the contact surface can bedefined in relation to the underside of the screw head or the minimumradius of the screw seat.

It is preferable for the annular contact surface to be located in theouter half of the underside of the screw head. More preferably theannular contact surface is located within the outer 75% of the undersideof the screw head, and even more preferably in the outer 80%. In thecontext of the present invention the underside of the screw head isdefined as the surface which extends radially beyond the outer boundaryof the screw shaft to the maximum radius of the bottom end of the screw(R₂−R₁).

Preferably the minimum radius of the annular contact surface is at least20% greater than the minimum radius of the screw seat, more preferably25% greater. Preferably the contact surface is located within a range of125-150% of the minimum radius of the screw seat. In a particularlypreferred embodiment the contact surface is located within a range of128-140% of the minimum radius of the screw seat.

Preferably, in use, at least the inner 50% of the surface area of thescrew seat is not contacted by the annular contact surface.

The above ratios provide a suitable inner area of uncontacted screw seatin order to provide an effective increase in the friction radius,without requiring any increase in the overall diameter of the screw heador screw seat.

As discussed above, in many embodiments the minimum radius of the screwseat is approximately equal to the maximum radius of the screw shaft.

Consequently, in a preferred embodiment the minimum radius of theannular contact surface is at least 20% greater than the maximum radiusof the screw shaft, more preferably 30% greater. Preferably the contactsurface is located within a range of 125-150% of the maximum radius ofthe screw shaft. In a particularly preferred embodiment the contactsurface is located within a range of 130-140% of the maximum radius ofthe screw shaft.

In the field of dental implants, in which the screw component has verysmall dimensions in order to fit within the implant, the maximum radiusof the screw shaft, R₁, is preferably between 0.6 and 1 mm and themaximum radius of the bottom end of the screw head R₂ is preferablybetween 1.2 to 1.5 times R₁. A particularly preferred range for R₂ is0.8-1.3 mm.

As mentioned above, it is desirable for the annular contact surface tobe narrow, and therefore in some embodiments the distal end(s) of theprotrusion(s) may be pointed or curved in shape. Upon tightening of thescrew head against the screw seat, such a narrow contact surface wouldbe deformed and flattened against the screw seat. This ensures a veryclose contact between the two surfaces and is particularly beneficialwhen the surface of the screw seat is rough or uneven. In many caseshowever it is preferable that the one or more protrusion comprises aflat distal end. This is easier to manufacture and reduces the risk ofinjury to the user. Preferably the contact surface has a radial width of10-20% of the maximum screw shaft radius and/or 10-20% of the minimumscrew seat radius.

In the field of dental implants, the contact surface has a width ofpreferably between 0.05-0.15 mm. Preferably the difference between themaximum radius of the screw shaft and the minimum radius of the annularcontact surface is between 0.2 and 0.4 mm.

The at least one protrusion can be formed such that this extendsdownwardly at approximately right angles from the underside of the screwhead. However, preferably the one or more protrusion is tapered at leaston its radially inner side.

Such a taper brings a further advantage to the present invention. As thescrew is tightened against the screw seat the tapered protrusion flexesslightly in the upward, or coronal, direction.

This is particularly beneficial in 3-part systems, such as when aseparate screw component is used to attach a dental abutment to animplant. Over time and with use, it is common for the abutment to settleslightly into the implant. In prior art systems this results in thescrew seat sinking away from the screw head and hence in a reduction inthe friction between the components. This is evidenced by the removaltorque necessary to unscrew the screw after use or dynamic testing,which is always significantly less than the initial insertion torqueused.

It has been surprisingly found however, that in addition to increasingthe maximum torque which can be withstood by the screw during insertion,the screw design of this preferred embodiment also increases the removaltorque. This is considered to be due to the above mentioned flexing ofthe screw head. This enables the screw head to act as a spring and, asthe abutment sinks during use, the screw head unflexes and lowers withthe abutment such that a greater degree of contact is maintained betweenthe screw seat and screw head. This therefore increases the removaltorque required to unscrew the screw and thus increases the security ofthe connection.

Preferably the radially inner side of the one or more protrusion tapersdownwards at an angle of between 15 and 25°, most preferably 20°.Preferably the taper is at least partially, preferably fully, curvedover a radius. When the taper is only partially curved this curvatureshould preferably be located at the proximal end of the protrusion.

The axial location of the bottom end of the screw head is defined by theannular contact surface, which in accordance with the present inventionis radially separated from the inner part of the screw head by a gap.The radially inner part of the bottom end of the screw head joins to thescrew shaft. This part of the screw head can have a radius equal to R₁,or in some cases greater than R₁, as long as this is less than theminimum radius of the screw seat.

Preferably the screw comprises an undercut at the transition between thescrew shaft and the screw head, such that the taper of the protrusionstarts radially inwards of R₁. This enables the tapered protrusion tohave a longer radial length, which in turn increases the spring effect.When the taper is curved over a radius this curve preferably continuesinto and forms at least a part of the undercut.

The undercut can be located within the screw shaft or screw head orboth. In a preferred embodiment undercut is at least partially locatedin the screw shaft such that the upper end of the screw shaft has aradius less than R₁. This increases the tolerance between the screwshaft and the screw seat edge.

The provision of a tapered protrusion on the screw head is consideredinventive in its own right and therefore, viewed from another aspect thepresent invention provides a fastening system comprising a screwcomponent and a seating component. The screw component comprises a shaftextending along a longitudinal axis and having a maximum radius R₁, saidshaft comprising a threaded section, said screw component furthercomprising, at one end of the screw shaft, a head, said head comprisinga bottom end having a larger maximum radius R₂ than the screw shaft suchthat an exposed underside of the screw head extends radially beyond themaximum radius of the screw shaft. The seating component comprises ascrew channel comprising a planar screw seat having a minimum radius R₃equal to or greater than R₁. The bottom end of the screw head comprisesan annular contact surface for abutment against this screw seat, theannular contact surface being formed by the distal end of at least onedownwardly extending protrusion on the underside of the screw head,wherein said at least one downwardly extending protrusion is tapered atleast on its radially inner side.

Preferably the minimum radius of the contact surface is greater than theminimum radius of the screw seat. The screw component in accordance withthis aspect may additionally or alternatively have any or all of thepreferred features discussed herein.

It has been found that, using a screw component in accordance with thepresent invention the maximum torque that can be withstood by the screwcomponent can be increased up to 10%.

In accordance with the present invention therefore, the incidence rateof screw failure can be reduced without needing to make externaldimensional or material alterations to the system. No alterations to theshape or volume of the clamped components, such as the implant andabutment, are required.

Preferably the screw component is integrally formed.

The screw and seating components of the present invention can be used inany technological field in which standard flat head screws are used. Theinvention is particularly beneficial in systems in which there islimited ability to alter the external dimensions of the clampedcomponents, such as dental implant systems. Therefore, preferably thescrew component is a dental screw component. This can be, for example, asecondary dental component, such as a dental abutment, for directattachment to a dental implant or other dental component. Preferablyhowever the screw component is a dental screw arranged to attach onedental component, e.g. an abutment or other secondary component, toanother, e.g. implant. The dental screw could also be used to attach,for example, a prosthesis to an abutment.

For the avoidance of doubt, a dental screw is an element which is usedto clamp one component to another. It therefore can be seen as a “thirdcomponent” of the system. The dental screw does not itself perform anyfunction in the dental implant system other than to attach anothercomponent to the system via clamping. In contrast, when the screwcomponent of the present invention is a secondary dental component, thisperforms an additional function once attached to the implant. Forexample, an abutment provides a support structure for the prosthesiswhile a healing cap seals the implant during osseointegration andassists in shaping the gingiva around the implant. A prosthesis providesa temporary or permanent replacement to a natural tooth or teeth.

In one preferred embodiment the screw component comprises a dentalsecondary component, such as an abutment and the seating componentcomprises a dental implant. In this embodiment the screw channel isformed by an interior bore in the implant. The planar screw seat may beformed within this bore or may be formed by the coronal end face of theimplant, i.e. at the coronal end of the screw channel.

In another preferred embodiment the seating component comprises a dentalsecondary component and the screw component is a dental screw forsecuring the secondary component to an implant. In such embodiments thescrew seat is usually located within the screw channel, which runsthrough the secondary component. The secondary component can be, forexample, a dental abutment or an impression post.

Preferred embodiments of the invention shall now be described, by way ofexample only, with reference to the accompanying drawings, in which:

FIG. 1A shows a basal screw of the prior art;

FIG. 1B shows a detail X from FIG. 1A

FIG. 2A shows a basal screw in accordance with the present invention;

FIG. 2B shows a detail X from FIG. 2A;

FIG. 3A shows a schematic representation of the screw of FIG. 1 incontact with the screw seat of a secondary component;

FIG. 3B shows a schematic representation of the screw of FIG. 2 incontact with the same screw seat; and

FIGS. 4-6 show alternative seat head designs according to the presentinvention;

FIG. 7 shows a base view of a screw component in accordance with thepresent invention;

FIG. 8 shows a base view of a screw components in accordance withanother embodiment of the present invention; and

FIG. 9 shows an abutment in accordance with the present invention.

FIGS. 1A and B show a basal screw 1 in accordance with prior artsystems. It comprises a screw shaft 2 extending along a longitudinalaxis 5. At its distal end the shaft 2 comprises a threaded section 3,the size and pitch of the thread being chosen for engagement with theinternal threaded bore of an implant. At its opposing end shaft 2 joinsto the screw head 4. Head 4 contains a hollow 6 in its coronal end whichis shaped to allow insertion of a drive tool such as a screw driver.Hollow 6 has a non-circular symmetric outline such that torque can betransmitted from the drive tool to the screw 1.

The bottom, or apical, end of head 4 has a larger maximum radius R₂ thanthe maximum radius R₁ of the shaft 2. This results in an annular contactsurface being formed by the underside 8 of the screw head 4. As theabutment or other component with which the screw engages must comprise ascrew channel dimensioned to allow passage of the screw shaft 2 themaximum contact area possible between the screw head 4 and screw seat isn(R₂ ²−R₁ ²).

It is worth noting that the maximum radius R₂ of the bottom end is lessthan the overall maximum radius of the screw head 4. This is because abevelled surface 9 links the bottom end to the outer circumference ofthe screw head 4. This improves the fit of the screw 1 within the screwchannel, as will be demonstrated later.

Despite this bevel, underside 8 provides a relatively large surface areawith which the head 4 can contact the screw seat of the abutment orother secondary component. Further, this surface area extends from themaximum radius R₁ of the screw shaft 2 outwards.

FIG. 2A shows a screw 10 in accordance with the present invention. Thescrew shaft 12 is identical to that of the prior art screw shown in FIG.1 and comprises a threaded section 13 at its distal end and joins to ascrew head 14 at the opposite end. Once again, head 14 comprises ahollow 16 shaped to allow engagement with a drive tool. Although shownat the distal end threaded section 13 could alternatively be positionedat a different axial location on the shaft 2.

In contrast to the prior art, the underside 18 of screw head 14 is notplanar but instead comprises a downwardly extending protrusion 17. Thisprotrusion 17 tapers downwards to a flat distal surface, which definesthe bottom, or apical, end 15 of the screw head 14. The taper is formedon the radially outer side by bevelled surface 19 and on the radiallyinner side by a concave surface.

The shape of the underside 18 results in the creation of an annularcontact surface having a width less than R₂−R₁ and which is locatedtowards the outer radius of the screw head 14. This shape of screw headincreases the friction radius of the screw and hence increases thetorque required in order to overcome the frictional resistance of thescrew head.

This is demonstrated with reference to FIGS. 3A and B. FIG. 3A showsschematic view of a partial cross section of the screw 1 of FIG. 1within a 3-part implant system. Abutment 20 comprises a screw channel 21having a coronal part and an apical part, separated by a step change indiameter which forms screw seat 22. Screw seat 22 is planar andperpendicular to the longitudinal axis 5 of the system. Due tomanufacturing methods the transition from the outer wall of the coronalpart of the screw channel 21 to the seat 22 is curved.

Abutment 20 is seated in an internal bore 31 of implant 30. The bore isshaped to snugly accommodate the abutment 20 and comprises a threadedsection 33.

In order to connect the abutment 20 to the implant 30 screw 1 is passedthrough screw channel 21 until the threaded section 3 of the screw 1 canengage with the threaded section 33 of the implant. By tightening screw1 the head 4 is forced down onto the screw seat 22 and clamps theabutment 20 within the implant 30.

The bevelled edge 9 of screw head 4 prevents any interference with thecurved transition area of the screw channel 21. The planar surface ofthe underside 8 creates a broad contact region C₁ between the screw head4 and screw seat 22. The friction radius of the system shown in FIG. 3Ais the mean radius of this contact region R_(F1).

When torque is applied to the screw 1 via the hollow 6 (not shown inFIG. 3A) a part of this torque will be used to overcome the frictionalresistance under the screw head 4, another part will be used overcomingthe frictional resistance of the screw threads and the remainder willtighten the screw and increase the tension in the screw body. Too muchapplied torque will over tension the screw 1 and cause this to fractureand break.

FIG. 3B shows the same implant system as FIG. 3A, however this timescrew 10 is used to connect the abutment 20 to the implant 30. As can beseen, the downwardly extending protrusion 17 significantly reduces thecontact region C₂ between the screw head 14 and the screw seat 22.

Significantly, no contact between the surfaces exists at the radiallyinnermost area of the screw seat 22 because the contact surface of thescrew head has a greater minimum radius than the minimum radius R₃ ofthe screw seat 22. This radius is similar to that of the maximum radiusR₁ of the screw shaft 12, as the screw shaft 12 must be able to passthrough the screw seat 22 into the apical part of the screw channel 21.

The lack of contact at the radially inner area of the screw seat 22increases the friction radius R_(F2) of the system and consequently thetorque needed to overcome the frictional resistance under the screw head14. By using a screw in accordance with the present invention thereforea smaller percentage of the applied torque will be used to tension thescrew body and hence the screw 10 can withstand more torque before overtensioning occurs.

Protrusion 17 is located as close to the outer radial edge of the screwhead 14 as possible, in order to increase the friction radius R_(F2). Inaddition the distal surface of protrusion 17 is made as narrow aspossible.

The protrusion 17 of the screw 10, shown in FIGS. 2 and 3B has a taperedsurface on its radially inner side. This has an additional benefit as itallows the screw head 14 to flex. As the screw head 14 is drawndownwards onto the screw seat 22, the tapered surface enables theprotrusion to pivot slightly. The screw head 14 thus acts as a loadedspring. During use of the abutment 20, this settles or sinks furtherinto the implant bore 31. In prior art systems this lessens the torquerequired to remove the screw 1. Using a screw according to a preferredembodiment of the present invention however, as the abutment 20 settleslower in the implant 30 the tapered protrusion 17 unflexes and hencemaintains a better contact with the screw seat. This leads to a higherremoval torque even after prolonged use of the abutment 20.

In order to increase the length of this taper, the screw 10 comprises anundercut 11 at the transition from the screw shaft 12 to the screw head14. This increases the spring effect of the protrusion 17 and inaddition increases the tolerance between the screw head 14 and screwseat 22. In this embodiment it is the curve of the taper which continuesinto and forms a part of the undercut 11.

Comparative tests have been run on screws having the designs shown inFIG. 1 and FIG. 2. It was found that the average breakage torque wasincreased from 51.8 Ncm, in the case of screw 1, to 57.8 Ncm in the caseof screw 10. In addition, after fatigue testing with a load of 280 N,the removal torque of screw 10 was 26.1Ncm compared with 17.9 Ncm inrespect of screw 1.

FIGS. 4-6 show some further embodiments of the present invention. FIG. 4shows a screw head 44 having a tapered protrusion 47 that tapers to anend point which forms the apical end 45 of the head 44, located at theouter radius of the screw head. This screw 40 provides the optimumfriction radius possible for a screw of a given outer radius and can beused in situations in which manufacturing tolerances permit, forexample, when the screw seat 22 is formed by the outer surface of theseating component and/or extends radially beyond the screw head. Thisembodiment features an undercut 41 which increases the spring effect ofthe tapered protrusion 47. This undercut 41 is located in the screw head44.

FIG. 5 shows an alternative screw head 54 in which protrusion 57 extendsat right angles from the underside 58 of the head 54. Providing a flatapical end 55 and contact surface enables better consistency andpredictability of the screw. In this embodiment no undercut is presentat the transition between the shaft and head.

FIG. 6 shows a screw head 64 having an undulating underside 68 thatresults in a curved protrusion 67. Here, the undercut 61 extends intoboth the screw shaft 62 and screw head 64.

FIG. 7 shows a generalised bottom view of a screw according to thepresent invention. Screw head 74 has a greater radius than screw shaft72 and so extends outwards from this forming an underside 78. Bevelledsurface 79 extends between the radial edge of the underside 78 and thecircumferential edge of screw head 74. The underside 78 of the screwhead 74 comprises a protrusion which extends in the apical direction toa distal end surface which forms a continuous annular contact surface C.The location, width and shape of the protrusion can vary, as shown inFIGS. 2 and 4-6. In FIG. 7 a single protrusion extends 360° about thelongitudinal axis of the screw to form a uniform, continuous contactsurface.

It is also possible for the contact surface to be formed by a pluralityof protrusions. This is shown in FIG. 8. Here it can be seen that theunderside 88 of the screw head 84 comprises multiple protrusions 87,which again can have any of the shapes shown in previous embodiments,each extending apically to a distal surface, these distal surfaces incombination forming a broken or discontinuous annular contact surface C.

The invention has mainly been described above in relation to a separatescrew component, which can be used to connect a secondary component suchas an abutment to an implant. However, it is also possible for thesecondary component itself to form the screw component of the presentinvention. When it is not necessary to know with certainty the exactangular position of the component with respect to the implant thesecondary component is often directly screwed into the implant. Thisdirect connection is common for example, when the implant is intendedfor supporting a bridge, i.e. a single prosthesis which replacesmultiple teeth. In such situations the bridge is attached to two or moreimplants and the angular orientation of the bridge is thus defined bythese multiple connection points. Other secondary components, such ashealing caps, which are only used on a temporary basis and do notsupport a prosthesis, may also be directly screwed to the implant.

FIG. 9 shows a secondary component designed for direct connection to animplant. The component 90 comprises shaft 92 having a threaded section93 for threaded connection to the implant. The component 90 furthercomprises a head 94, which in use protrudes from the implant into and/orthrough the soft tissue.

The bottom or apical end the head 94 has a larger radius than the shaft92, such that an underside 98 is created. The underside 98 comprises anapically extending protrusion 97 which extends 360° about thelongitudinal axis of the component such that an annular contact surfaceis formed. The detail circled in FIG. 9 is very similar in configurationto the screw 10 shown in FIG. 2A. The annular contact surface ofcomponent 90 has a larger minimum radius than the maximum radius of theshaft 92 and the minimum radius of the screw seat of the implant.Therefore, the friction radius of the screw head is increased inrelation to prior art components.

The above described embodiments are for illustrative purposes only andthe skilled man will realize that many alternative arrangements arepossible which fall within the scope of the claims.

Where technical features mentioned in any claim are followed byreference signs, those reference signs have been included just for thesole purpose of increasing intelligibility of the claims andaccordingly, such reference signs do not have any limiting effect on thescope of each element identified by way of example by such referencesigns.

1. A fastening system comprising a screw component and a seatingcomponent, the screw component comprising a shaft extending along alongitudinal axis and having a maximum radius R₁, said shaft comprisinga threaded section, said screw component further comprising, at one endof the screw shaft, a head, the head comprising a bottom end having alarger maximum radius R₂ than the screw shaft such that an exposedunderside of the screw head extends radially beyond the maximum radiusof the screw shaft, the seating component comprising a screw channelcomprising a planar screw seat having a minimum radius R₃, which isequal to or greater than R₁, wherein the bottom end of the screw headcomprises an annular contact surface (C) for abutment against said screwseat, said contact surface being formed by the distal end of at leastone downwardly extending protrusion on the underside of the screw head,the annular contact surface having a minimum radius greater than theminimum radius of the screw seat.
 2. A fastening system as claimed inclaim 1, wherein the annular contact surface (C) is formed by a singleprotrusion extending 360° about the longitudinal axis.
 3. A fasteningsystem as claimed in claim 1, wherein the annular contact surface (C)has a uniform inner radius.
 4. A fastening system as claimed in claim 1,wherein the annular contact surface (C) is located in the outer half ofthe underside of the screw head.
 5. A fastening system as claimed inclaim 1, wherein the minimum radius of the annular contact surface (C)is at least 25% greater than the minimum radius R₃ of the screw seat. 6.A fastening system as claimed in claim 5, wherein the contact surface(C) is located within a range of 128-140% of the minimum radius R₃ ofthe screw seat.
 7. A fastening system as claimed in claim 1, whereinminimum radius of the annular contact surface (C) is at least 30%greater than the maximum radius R₁ of the screw shaft.
 8. A fasteningsystem as claimed in claim 1, wherein the one or more protrusioncomprises a flat distal end.
 9. A fastening system as claimed in claim1, wherein the one or more protrusion is tapered at least on itsradially inner side.
 10. A fastening system comprising a screw componentand a seating component, the screw component comprising a shaft (12)extending along a longitudinal axis and having a maximum radius R₁, saidshaft comprising a threaded section, said screw component furthercomprising, at one end of the screw shaft, a head, said head comprisinga bottom end having a larger maximum radius R₂ than the screw shaft suchthat an exposed underside of the screw head extends radially beyond themaximum radius of the screw shaft, the seating component comprising ascrew channel comprising a planar screw seat having a minimum radius R₃equal to or greater than R₁, said bottom end of the screw headcomprising an annular contact surface (C) for abutment against theplanar screw seat, said annular contact surface being formed by thedistal end of at least one downwardly extending protrusion on theunderside of the screw head, said at least one downwardly extendingprotrusion being tapered at least on its radially inner side.
 11. Afastening system as claimed in claim 9, wherein the radially inner sideof the one or more protrusion tapers downwards at an angle of between 15and 25°, most preferably 20°.
 12. A fastening system as claimed in claim9, wherein the taper is at least partially curved over a radius.
 13. Afastening system as claimed in claim 9 wherein the screw componentcomprises an undercut at the transition between the screw shaft and thescrew head, such that the taper starts radially inwards of R₁.
 14. Afastening system as claimed in claim 12 wherein the screw componentcomprises an undercut at the transition between the screw shaft and thescrew head, wherein the curve of the taper continues into and forms atleast part of the undercut.
 15. A fastening system as claimed in claim1, wherein the screw component is a dental screw for attaching anabutment or other secondary component to a dental implant.
 16. A screwcomponent for use in the fastening system as claimed in claim
 1. 17. Ascrew component comprising a shaft extending along a longitudinal axisand having a maximum radius R₁, said shaft comprising a threaded sectionsaid screw component further comprising, at one end of the screw shaft,a head, said head comprising a bottom end having a larger maximum radiusR₂ than the screw shaft such that an exposed underside of the screw headextends radially beyond the maximum radius of the screw shaft, saidbottom end comprising an annular contact surface (C) for abutmentagainst a planar screw seat, said contact surface being formed by thedistal end of at least one downwardly extending protrusion on theunderside of the screw, said contact surface having a minimum radiusgreater than the maximum radius of the screw shaft.
 18. A screwcomponent as claimed in claim 17, wherein the minimum radius of theannular contact surface (C) is at least 30% greater than the maximumradius R₁ of the screw shaft.